Paper manufacture involves the blending of pulp (mainly wood fiber) with clay filler, retention aids such as calcium carbonate, latex dyes, and defoamers and running the slurry into a paper machine where it is formed into a sheet. In this process, some of the fiber and filler escape. The portions which escape and those portions which are unused in the papermaking process are collected by the paper mill in a paper sludge. The sludge is fed to a gravity thickener (if required to concentrate the sludge) and then to a dewatering device (usually a belt filter press, vacuum filter or screw press) which reduces the water content to approximately 50% to 80%.
Previously in the papermaking industry, waste paper sludge was burned as a part of the firing for the digesting process. The burning of the sludge is not particularly efficient because of its water content and because of its high ash content. Furthermore, the fiber content of the sludge further has a low heat value. Finally, steam generated by power boilers burning the sludge has a low intrinsic heat value.
One patent does disclose a method for recycling paper sludge. U.S. Pat. No. 5,137,599 to Maxham teaches a process for converting pulp and paper mill waste solids into a papermaking fibrous pulp of substantially the same quality as used by the pulp and paper mill to make its paper products. In this process, papermaking sludge is defibered to release individual fibers, screened to separate long fibers from short fibers, centrifugally cleaned to separate debris from the long fibers, bleached to increase the brightness of the fibers, dewatered to remove excess water from the pulp, clarified to separate short fibers and clay from the defibering effluent, and biologically treated to remove dissolved organic materials from the excess water generated which is either discharged from the process or recycled as process water.
U.S. Pat. No. 5,137,599 to Maxham fails to disclose a method in which the recovered cellulosic fibers are recycled back into the recovery process to generate heat for a dryer which dries the sludge.